In fuel cells, when the electrical circuit leading to a powered device, such as an automobile drive system, is opened during shut down of the cell and the cell is relieved of an electrical load, the presence of air on the cathode, coupled with hydrogen fuel remaining on the anode, causes unacceptable anode and cathode potentials, resulting in corrosion in the catalyst and the catalyst assembly support and consequent degradation of the cell assembly and the cell performance. Conventionally, an inert gas is used to purge both the anode flow field and the cathode flow field immediately upon cell shut-down to dissipate electrical potential at the anode and cathode. In a motor vehicle, an on board supply of inert gas such as nitrogen for use in the shut down purge adds weight and cost and may interfere with vehicle shut-down and start-up procedures.
In another aspect of a fuel cell system shut down procedure, hydrogen at the anode side, air at the cathode side, and generated water remain after shut down when the power consuming circuit is disconnected. An open circuit voltage occurs between the anode and cathode (about 1 volt for each cell) and remains until the residual fuel, hydrogen, and air are consumed. A post shut down prolonged voltage damages the membrane and electrode assembly (MEA). The voltage causes the platinum catalyst at the cathode side of the cell to be ionized; the platinum migrates into the membrane and generates radical or charged molecular elements that damage the molecular structure of the membrane.
In one proposed solution, the hydrogen remaining at the anode side is consumed to generate electricity, which is charged to a capacitor through a voltage converter. Remaining water at the cathode side is blown off with air by a pump. When the vehicle water pump is stopped, coolant remains in the fuel cell stack and cooling system, but, nevertheless, some hydrogen still remains at the anode side and produces a persistent voltage after a lapse of several hours. The persistent voltage in an inoperative cell will cause membrane damage. U.S. Pat. No. 6,858,336, Procedure for Shutting Down a Fuel Cell System Using Air Purge, proposes to shut down an operating fuel cell system by disconnecting the primary load device, stopping the flow of hydrogen fuel to the anode, and displacing the fuel remaining in the anode fuel flow field with air by blowing air through the anode fuel flow field. The '336 patent terminates the hydrogen flow and quickly displaces the remaining hydrogen by blowing air through the anode field at shut down. Generated heat in the fuel cell that causes hydrogen and air to react at the mixture front may cause some damage locally when this procedure is employed.
U.S. Pat. No. 6,391,485, Method and Apparatus for Purging a Fuel Cell System With Coolant, describes a fuel cell startup-shutdown method and apparatus for purging a fuel cell stack with coolant, typically water, during the transient operations of start-up and shutdown [Column 6, lines 27-43; Column 2, lines 43-46]. U.S. Pat. No. 7,090,940, Freeze Tolerant Fuel Cell Power Plant With a Direct Contact Heat Exchanger, relates to a freeze tolerant fuel cell and discloses a power plant including a coolant inlet and a coolant outlet for directing a coolant to flow through the fuel cell during a shutdown. [Column 2, lines 28-31; Column 6, lines 32-48]. Publication Number 2006/0040140, Yu et al., Feb. 23, 2006 and Publication Number 2007/0015018, Tsutsui, Jan. 18, 2007, describe systems for reducing the temperature of a fuel cell stack during the shutdown process. Publication Number 2007/0031713, Cho et al., Feb. 8, 2007, and Publication Number 2007/0128474, Bach et al., Jun. 7, 2007, respectively, relate to fuel cell cooling systems to reduce corrosion and cool the hydrogen gas flow, not the fuel cell itself, and to a system to provide cooling to reduce hydrogen depletion.